PARIS/WASHINGTON (Reuters) – In early spring France’s ambassador to the United Nations dined with a Russian colleague and discussed the crisis in Syria.

Ambassador Gerard Araud told the Russian diplomat France was going to go public with proof from its intelligence services that Syria’s government was using chemical weapons against its own people. The Russian diplomat laughed, according to a source familiar with the meeting. “Gerard,” he told his counterpart, “don’t embarrass the Americans.”

It was a revealing exchange. France and Britain had been pressing for almost a year for the United States to engage more directly in the war in Syria, where President Bashar al-Assad’s battle against a popular insurgency has killed 100,000 people and displaced more than 6 million. But Washington had resisted pleas for action, reluctant to get sucked into another Middle East quagmire after a decade of fighting and misadventure in Iraq and Afghanistan. It had no desire for France to pile on further pressure by telling the world Assad was committing atrocities with weapons of mass destruction.

Even when the French went public with their claims in early June, the Obama administration said it needed more time and evidence to judge what had happened. A couple of weeks later the White House said that U.S. intelligence agencies had “high confidence” that Assad had launched small scale chemical attacks at various points over the previous year. But while Paris said all options were on the table, Washington played down the attacks, merely promising to give more aid to the anti-Assad rebels in Syria.

The gap between the two Western allies was just one awkward step in an extraordinary two-year dance around the civil war in Syria. That dance, detailed here with reporting drawn from interviews with senior diplomats and officials over the past year, has grown ever more complicated in recent weeks after graphic evidence of a much bigger chemical attack hit computer and television screens around the world on August 21.

Videos posted online after the attack showed hundreds of people in suburbs of the Syrian capital Damascus struck by a mysterious, lethal affliction. Men, women and children struggled for breath, foaming at the mouth and twitching. Other scenes showed scores of corpses with no obvious wounds.

Rebels said Assad had killed hundreds of civilians with chemical weapons. Assad denied it, but the evidence suggested otherwise.

In the first few days after the attack it appeared likely that the United States and some of its allies would launch airstrikes on Assad and his military. In 2012, Obama had called a chemical attack in Syria a “red line” that should not be crossed.

But as the U.S. president began trying to convince Congress to back military strikes, the lack of political enthusiasm became obvious – and not just in Washington.